Japan is still waiting for a response from Beijing concerning an alleged intrusion last week by a Chinese nuclear-powered submarine into Japanese waters.
Japan's trade minister says he believes that a Chinese submarine, which Tokyo says intruded into Japanese waters last week, is linked to gas exploration by China in a remote island area claimed by both countries.
Trade Minister Shoichi Nakagawa, appearing on a Fuji TV network program Sunday, added he believes that Chinese submarines have been moving in Japanese waters for some time.
Japan on Friday demanded an apology from China. It claims a Han-class nuclear submarine spent two hours in Japanese waters on Wednesday. Japan mobilized its maritime forces and chased the sub with destroyers and a patrol plane as it zigzagged submerged toward Chinese waters.
China has not acknowledged the submarine, so far, only saying it is investigating the Japanese claim. But China's ambassador on Saturday, at a seminar in western Japan, called for the two countries to work together to resolve their mutual troubles.
Ambassador Wang Yi, without mentioning the submarine incident specifically, says China and Japan should respect each other and calmly find a solution.
Earlier this year, China began exploring for natural gas near the boundary of its exclusive economic zone. Japan has protested the exploration, saying it is concerned that China will also extract gas from the Japanese side of the zone.
Talks began last month between the two countries in an attempt to resolve the matter.
Japan, China and Taiwan all claim possession of a speck of islands called Senkaku in Japanese and Diaoyu in Chinese. They are about 500 kilometers from Japan's Okinawa Island and 140 kilometers from Taiwan.